A safety circuit in an elevator system and a method for monitoring semiconductor switches of an elevator system are known from WO 2011/054674 A1. A periodic measurement of the voltage or the current strength at the input and at the output of the semiconductor switches and, if the measurement revealed a short circuit, opening of the series connection of the safety circuit by means of a relay contact takes place with the known safety circuit and the known method. In other words, the elements of the electromechanical relay circuit of the known embodiment are used, in the case of a short circuit of the semiconductor switch, to open the safety circuit. The monitoring can take place here by means of a monitoring switching circuit which is processor-controlled. For the prevention or detection of a short circuit in a semiconductor switch, complicated and cost-intensive solutions can thus be avoided.
When use is made of a safety circuit, such as is known from WO 2011/054674 A1, the problem arises that the system must be in operation in order to carry out the check. If the energy supply is not available over a short or longer period at least in respect of the safety circuit, faults likewise cannot be detected. Such faults that have occurred in the meantime are then detected during or shortly after the re-establishment of the energy supply and then possibly lead to the elevator system having to be put out of operation for safety reasons. For the purpose of illustration, the following problem may for example arise. After the elevator system was temporarily put out of operation for the purpose of energy-saving, people enter the elevator car and select a target floor. When the elevator is intended to perform this travel command, the safety circuit is again energized. An emergency shutdown thus occurs when the elevator system is put back into operation or shortly thereafter if a corresponding defect has occurred during the stoppage of the elevator system and is now detected. In an unfavorable case, in which the fault is detected via the safety circuit only after the start of travel, people may get stuck in the elevator car.
A monitoring system for establishing a change in a switching position of a safety switch is known from WO2014/124779, wherein an energy supply device independent of the external energy supply is used in order to establish a change in a switching position of a safety switch even when the external energy supply is absent. It remains an open question here as to how such a monitoring system can be checked for correct functioning.